Dahlke, Sandro (2012) Tropische Instabilitätswellen und Vermischungsprozesse in der oberen Wassersäule des östlichen tropischen Atlantiks. (Bachelor thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 29 pp.

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Abstract

In this study, microstructure and current speed data up to 550 m depth from 2011 are used to investigate the vertical turbulence dissipation rate (ε) - and current distribution. Data for this have been determined during the MERIAN research cruise MSM 18/2 and MSM 18/3. Equatorial turbulence dissipation rate timeseries at 10°W until 360 m depth beginn to show vertical structures of increased values from June on until July. An observation of the current speed points out that the equatorial Atlantic is dominated by a complicated zonal current system which is strongly influenced by the wind field. However, instabilities of the zonal flow field, especially caused by increased meridional shear, can generate Tropical Instability Waves (TIWs). They again interact with the equatorial current system. So TIWs are able to generate Rossby- and Yanai waves which radiate their energy downward and contribute to the deep dissipation signals. Sea surface temperature (SST) can trace the TIWs, which are thought to have an anticyclonic current structure and wavelengths of about 1100 km. They propagate westward at a speed of ca. 30 cm/s and are only present during June and August, which is also the time of deep mixing signals, and in the region west of 7°W. At the equator, both the u- and v-component of current speed show a distinct vertical pattern, so that dominant signals like the Equatorial Undercurrent (EUC) or the TIW-associated surface flow can be identified. Furthermore, baroclinic structures induced by other tropical instabilties and various equatorial waves are found in the current profiles. A comparison of the u- and v-component of current shear to deep turbulence distribution signals underneath 100 m depth shows the influence of the found current patterns on the mixing structure. Maxima in meridional shear correlate very well with increased ε values. On the other hand side, maxima in zonal shear are more likely found in regions of strong stratification, so that they do not contribute very well to the ε structure. This could be a hint that tropical instabilities and waves might play an important role in mixing of equatorial water masses.

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